This invention relates to a total reflection of X-ray fluorescence analyzing system, and more particularly to a system for allowing an X-ray to be incident below an angle of total reflection onto, e.g., a silicon (Si) wafer surface to measure a quantity of an excited fluorescent X-rays generated from metal impurity such as chrominum (Cr), iron (Fe), nickel (Ni), copper (Cu), aluminum (Al), or zinc (Zn), etc. on the surface of the wafer to analyze, on the basis of the measured result, the presence and absence of the surface metal impurity, a quantity of the attached surface metal impurity, the kind thereof, or the state of the distribution thereof, etc.
Since there are instances where metal impurities as stated above may be attached on the Si wafer in the manufacturing process, an approach is generally employed to analyze the metal impurity on the surface of the manufactured Si wafer to thereby carry out the quality control to seek for the cause of attachment thereof thus to take a measure to prevent the metal impurity from being attached.
At the beginning time when such a metal impurity analysis was conducted, an approach was employed to form a thermal oxide film of about 1000 angstroms on the Si wafer surface to take the metal impurity into the thermal oxide film to dissolve out, by acid, the thermal oxide film including the metal impurity from the Si wafer to analyze the impurity thus dissolved out by the atomic absorption method.
Thereafter, since, by the development of the analyzing method, it was become unnecessary to form a thermal oxide film so that the thickness thereof is kept at about the above-mentioned value, an approach was employed to dissolve out an oxide film including metal impurity of about 30 angstroms naturally grown on the wafer surface to analyze the dissolved oxide film including metal impurity.
However, in the case of the above-mentioned analyzing method, since hydrofluoric acid (HF) vapor or nitric acid (HNO.sub.3) is used as an acid for dissolving the oxide film, there exists the problem that there is the extremely high possibility that an operator may be harmed by vapor.
In view of this, an analyzing method was conventionally developed to allow X-ray to be incident onto the surface of the wafer to excite metal impurity atoms to measure fluorescent X-ray generated by excitation to carry out an analysis with respect to the surface metal impurity of a measurement sample on the basis of the measured result. The term "excitation" is used in a sense to give an energy to atoms to conduct a transition thereof from the ground state to a higher energy level.
Namely, it is known that when an X-ray is incident to the material, atoms are excited, whereby a fluorescent X-ray (scattered light) is generated in addition to a reflected light. The X-ray analyzing method utilize this phenomenon. Since the light quantity of the fluorescent X-ray is proportional to the quantity of an object to be excited, the quantity of the metal impurity attached can be measured by measurement of that light quantity. Further, since the metal impurity has an energy peculiar to the object to be excited, the kind of the metal impurity is made clear by examining that energy.
As stated above, in accordance with the above-mentioned analyzing method, since an approach is not employed in analysis to utilize the harmful gas mentioned above, there is no possibility that the health of an operator is injured.
In addition, in accordance with X-ray analyzing method, an excited X-ray is caused to be partially incident onto the wafer to thereby permit implementation of the positional analysis of the wafer in-place, i.e., the analysis of the state of the in-plane distribution of the metal impurity, or the analysis where the in-plane location of the metal impurity is designated. Thus, this X-ray analyzing method makes it possible to conduct a more detailed analysis as compared to the above-mentioned chemical analyzing method. Further, since there is no destruction of a wafer used as the sample, that wafer can be used as the material of the chip, and implementation of the in-line analyzing system can be realized. For this reason, at present, the approach where the metal impurity analysis on the wafer surface is conducted by the X-ray analyzing method is being the main current.
Meanwhile, the analyzing system for carrying out this analysis is roughly composed of an X-ray source, a spectroscope, and a detector, e.g., Solid State Detector (SSD).
X-ray from the X-ray source is changed to a monochromatic X-ray, and is incident onto the wafer below an angle of total reflection. The SSD is arranged so as to avoid that reflected X-ray, but receive only a fluorescent X-ray excited and generated. A light quantity value is provided by a count value of the SSD.
In accordance with the X-ray analyzing method, however, since the wafer subject to analysis is a single crystal, there are instances where the incident X-ray causes the Bragg reflection in dependency upon the incident direction of the X-ray, and the diffracted light thereof is incident to the detector. In this case, since many light quantity values due to the diffracted light which would be the cause of noise are included in measured light quantity value at this detector, there results the problem that the sensitivity is lowered.
Further, since when an extremely intensive X-ray is incident to the detector, the dead time of the detector is prolonged, there is the problem that the measurement time becomes longer than that required owing to the light quantity of that diffracted light.
In this connection, the intensity of an incident light to the detector when the Bragg reflection takes place is 10 to 20 times larger than that when no Bragg reflection takes place. As a result, the measurement time in the former case is 2 to 10 times larger than that in the latter case.
As described above, the conventional total reflection of X-ray fluorescence analyzing system has the problem that lowered sensitivity or prolonged measurement work is caused by the Bragg reflection.